As is known in the art, exhaust gas recirculation (EGR) systems are employed in automotive vehicles in order to help reduce engine emissions and improve fuel economy. Such systems typically employ an EGR valve that is disposed between the engine exhaust manifold and the engine intake manifold, and operable, when in an open position, to recirculate a portion of the exhaust gases from the exhaust side of the engine back to the intake side. In one arrangement, the EGR flow rate to the intake manifold is varied according to one or more sensed conditions, such as engine temperature, air charge entering the intake manifold, and engine speed. In one type of EGR control system, electrically actuated EGR valves have also been introduced which use software-implemented control logic. Such control logic controls input to an electric actuator motor which, in turn, positions the EGR valve. In such systems, the control logic may generate pulse width modulated (PWM) signals to power the actuator motor, and modulate the acceleration and deceleration of the EGR valve as it moves to its desired positions.
An open loop system is generally faster and less expensive than a closed loop system, but requires a separate device to diagnose failure. This other device is usually a manifold absolute pressure (MAP) sensor. The cost of the MAP sensor offsets much of the cost benefit of the open loop system. Open loop EGR systems typically use a stepper motor valve, which reliably moves the valve to a requested position. Flow through the valve is inferred by knowing the pressure before and after the valve in concert with its position. Unfortunately, open loop flow prediction degrades rapidly as particulates clog the valve, and requires the valve itself to have minimal variability in its manufactured flow characteristics.
Closed loop systems measure flow using pressures before and after a control orifice, which is located within the EGR flow path. The measured flow is compared to the requested flow. The valve is then moved to minimize flow error. This process has improved steady state performance in terms of the actual flow matching the requested flow, regardless of how degraded or variable the valve flow characteristic may be. However, this system is slower in reaching its final position. Furthermore, if used too aggressively to minimize the time response, closed loop systems can be unstable.
As is also known in the art, one technique used to provide absolute pressure is by inferring such pressure, i.e. an “inferred MAP”, from mass airflow (MAF) sensor readings. This technique is described in U.S. Pat. No. 5,505,179, issued Apr. 9, 1996, inventor Daniel G. Brennan, and U.S. Pat. No. 5,331,936, issued Jul. 26, 1994, inventors Messih et al, both assigned to the same assignee as the present invention, the entire subject matter of both such U.S. patent numbers being incorporated by reference.
The inventors have recognized that Manifold Absolute Pressure (MAP) downstream of the EGR flow into the intake manifold consists of three primary contributors: fresh air into the intake manifold upstream of the EGR flow into the intake manifold, EGR flow into the intake manifold, and various uncounted leakages. The amount of fresh air into intake manifold is sensed by the Mass Air Flow (MAF) sensor upstream of the EGR flow into the intake manifold and its contribution to MAP downstream of the EGR flow into the intake manifold plus some modeled leakages may be designated as the inferred manifold absolute pressures (INF_MAP) with zero EGR flow into the intake manifold (i.e., INF_MAP_with 0 EGR).
Thus, the actual manifold pressure downstream of the EGR flow into the intake manifold, (i.e., MAP_ACTUAL) is equal to INF_MAP_with—0_EGR plus pressure from the EGR flow into the intake manifold and pressure from all other sources into the intake manifold upstream of the EGR flow represented as a function of air charge (AIRCHG). That is,MAP—ACTUAL=INF—MAP—with—0—EGR+func1(AIRCHG)+func2(EGR—FLOW),                 where:        func1 is a first function;        func2 is a second function;        AIRCHG is cylinder air charge, where AIRCHG(LBS_AIR/Cyl_Fill)=AM (LBS_AIR/min)/(Engine_speed*Number_of_cylinders/2) and AM is measured by the MAF sensor; and        EGR_FLOW is the actual EGR flow into the intake manifold and is a function of a desired EGR flow rate, EGR_RATE_DES.        
Thus, the contribution of EGR flow to the actual MAP (MAP_ACTUAL) can be determined theoretically by calculating the difference between MAP_ACTUAL and INF_MAP_with—0_EGR, where INF_MAP with 0 EGR is may be calculated from MAP sensor readings, minus uncounted leakages. Therefore, due to certain degree of disagreement between the MAF and MAP sensor readings, hardware to hardware variations, and engine mapping limitation, the resulted difference between MAP_ACTUAL, as measured by the MAP sensor, and the INF_MAP calculate from MAF, excluding EGR contribution is very dynamic and difficult to model. Within a limited engine speed range and at carefully selected engine operation conditions, however, this difference MAP_ACTUAL−MAP_with 0_EGR may be represented as a linear function of cylinder aircharge and the contribution of uncounted leakages is mainly stationary.
More particularly, with the MAF sensor disposed upstream of the exhaust gas inlet to the intake manifold and the MAP sensor disposed downstream of the: inlet, the relationship below follows:MAP—ACTUAL INF—MAP—with—0—EGR=B0+B1*AIRCHG+B2*K*EGR—RATE_DES+Noise where:                MAP_ACTUAL is the output of MAP sensor;        INF_MAP_with—0_EGR is the INF_MAP with zero EGR, and determined as a function of MAF sensor readings;        B0 is an offset, determined from engine characterization data B1 is the linear slope for the variation of MAP_ACTUAL−INF_MAP_with—0_EGR with AIRCHG and reflects of difference between measured MAF and measured MAP as a function of AIRCHG and determined from engine characterization data;        B2 is the linear slope for the variation of MAP_ACTUAL−INF_MAP_with—0_EGR with the desired EGR flow and reflects the ratio of actual EGR flow into the intake manifold over the commanded, or desired EGR flow (i.e., EGR_RATE_DES) into the intake manifold. It is determined from engine characterization data. With a fully functional EGR system, B2 is equal 1 theoretically and the desired EGR flow is the same as the actual EGR flow into the intake manifold. Therefore, the estimated B2's value in this method reflects the level of degradation of EGR delivery system.        K is the normalization coefficient to ensure the above equation stands and B2 equals to 1 for a fully functional EGR system.        EGR_RATE_DES is the requested, or commanded, EGR rate. It determines signal sent to the EGR valve for regulating EGR flow into the intake manifold.        Noise is any neglected contributions. Within a pre-selected window, this term is insignificant; and        During characterization of the engine having an EGR valve that is known to be operating properly, data is taken and the coefficients B0, B1 and B2 are determined using for example, least mean square fitting. The parameter B0, B1, and B2 are here estimated by a Recursive Least Squared technique. During normal engine operation, measurements of MAF and MAP are taken every 100 milliseconds for example, and is used to determine MAP_ACTUAL minus INF_MAP_with—0_EGR as a function of cylinder aircharge and desired EGR flow. Using a linear regression on such data and the relationship:MAP—ACTUAL−INF—MAP—with—0—EGR=B0—ACTUAL+B1—ACTUAL*AIRCHG+B2—ACTUAL*K*EGR—RATE—DES+Noise the coefficients B0_ACTUAL, B1_ACTUAL and B2_ACTUAL are thereby determined. The determined coefficient B2 ACTUAL is compared with the coefficient B2 determined during engine characterization to determine whether the difference between B2 and B2 ACTUAL is within a predetermined acceptable range. If not within such range, an indication is provided that the degradation of EGR system is beyond the acceptable level.        
Thus, in accordance with the present invention, a method is provided for determining EGR flow in an internal combustion engine, such flow being from an exhaust manifold of the engine to an intake manifold of the engine through an EGR valve. The method includes using information provided by a mass air flow sensor disposed upstream of an exhaust gas inlet to the intake manifold and information provided by an manifold absolute pressure sensor disposed downstream of such exhaust gas inlet to provide an indication of the flow of exhaust gas into the intake manifold through such inlet. The method compares such estimated exhaust gas flow into the intake manifold with a commanded exhaust gas flow to the EGR valve. With such method, a determination may be made as to whether the EGR valve is operating properly.
With such method, a dynamic equation, which relates the contribution of EGR flow to manifold pressure and an inherent difference between MAP_ACTUAL and INF_MAP, is established within a limited range of engine speeds and at selected engine operation conditions. A Recursive Least Squared technique is applied to estimate EGR flow contribution, which provides a desired means for the degradation detection.
In accordance with another feature of the invention, a method is provided for determining EGR flow in an internal combustion engine, such flow being from an exhaust manifold of the engine to an intake manifold of the engine through an EGR valve. The method includes: determining actual manifold absolute pressure readings from an manifold absolute pressure sensor disposed downstream of an inlet for the EGR flow into the intake manifold; obtaining readings of airflow into the intake manifold upstream of the EGR inlet; computing an inferred manifold absolute pressures from the airflow readings; obtaining samples of a desired EGR flow signal fed to a valve for controlling the EGR flow into the inlet; determining a coefficient B2, such coefficient being a function of: (A) the differences between the determined actual manifold absolute pressure readings and the calculated manifold absolute pressures; and (B) the obtained desired EGR flow samples; comparing the determined coefficient B2 with a predetermined value for B2.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.